The present invention relates to an optical head which optically reads and writes signals recorded on a recording medium, and in more detail, to an optical head in which a structure for detecting focusing error signals and tracking signals is improved.
Generally, the optical head detects a change of the focal point according to a change of a distance between an objective lens and a recorded medium so as to feed back the detected change to a location control system of the head. The structure for detecting such signals comprises a lens system having an objective lens which receives light reflected from the medium, a detector for receiving the light having passed through the lens system, and a light control system located between the lens system and the detector. Thus, the changed signal value is obtained in the detector according to the change of the distance between the objective lens of the lens system and the medium.
The lens system includes the objective lens and a detecting lens as basic components. The control system comprises a knife edge, as it is called, which is located along the central axis of the beam path, the above-mentioned detector, and a bisected photodetector on which the light passing through the knife edge is focussed. In such a structure, the light signal whose flux is divided into two parts along the central axis of the beam path at the knife edge is retransmitted through a beam splitter separately provided on the optical path between the lens system and the knife edge so as to compensate for astigmation. At the same time, the light signal is focussed around a laser diode chip being a light source so as to narrow the distance between the chip and the photo detector, thereby miniaturizing the optical system, which was possible only in the conventional hologram-type.
FIG. 1 schematically illustrates the structure of a focussing and tracking signal detector of the conventional optical head. In the structure, the process for generating light and detecting the focal point and the tracking signal will be explained from the following description.
A reference numeral "7" represents the medium, with "7a" representing a protective substrate and "7b" representing a record surface thereof. A reference numeral "6" represents the objective lens, while "5" represents a beam splitter inclined at a predetermined angle with respect to the optical axis of the objective lens. Successively, a reference numeral "9" represents a knife edge, "11" represents the focussing signal detection photodetector, "1" represents the laser diode chip facing the beam splitter, "2" represents a mount for supporting the chip, "3" represents a cover glass being a protective glass, "4" represents a housing for housing and protecting the components, and "10" represents a tracking signal photodetector provided on the optical axis separated by knife edge 9. In such a structure, photodetector 11 for detecting focussing errors should be located at the focal point of the incident light, and photodetector 10 for detecting the tracking signals is located at a point of deviation from the focal point.
A laser light which is generated from laser diode chip 1 being the light source transmits through cover glass 3 being a protective glass so as to be incident to beam splitter 5 which has a coated surface 5b on a light transmittable substrate 5a. Among beams incident to beam splitter 5, some beams passing through coated surface 5b pass substrate 5a, and other beams reflected from coated surface 5b are focussed as fine spots by objective lens 6 so as to transmit through protective substrate 7a of media 7, thereby being concentrated on record surface 7b. The beams reflected from record surface 7b transmit through protective substrate 7 and objective lens 6. Then, a portion of the reflected beam is reflected by coated surface 5b of beam splitter 5, while the remainder is transmitted through substrate 5a. Portions of this transmitted beam are again reflected by knife edge 9 with the remainder thereof transmitting through the knife edge. Here, focal point signals are detected by photodetector 11 which detects the partially separated beam, and tracking signals are detected by photodetector 10 which detects the reflected beams.
The drawbacks exist in such a conventional optical head. Firstly, since the distance from beam splitter 5 to photodetectors 10 and 11 must be more than a predetermined distance, miniaturization is thus limited. Secondly, two photodetectors are required since they should be respectively positioned at the focal point surface and at same point adjacent thereto. Given that photodetector 10 for detecting the tracking signals is located at a place other than the focal surface, when the light incident to photodetector 10 moves in addition to the normally varying tracking signal, the thus-changed amount of light is detected so as to appear as a tracking error.
That is, in the conventional focussing and tracking signal detection type, since the focussing detection photodetector is located at the focal point of the detected light, detecting the tracking error is impossible. Accordingly, in the optical head for detecting the focussing error having the conventional knife edge type structure, a detector for detecting the tracking error is separately provided, miniaturization is further limited. Also, since the optical head is composed of many components, its manufacture is difficult the unit cost is high.